The present invention generally relates to spindle unit control methods and image forming apparatuses, and more particularly to a spindle unit control method and an image forming apparatus which are suited for forming an image having a high quality.
For example, a spindle unit includes a polygonal mirror which is provided on a rotatable spindle, and by irradiating a light beam such as a laser beam on the rotating polygonal mirror, it is possible to optically write image data by scanning a photoconductive body by a reflected light beam from the polygonal mirror. Accordingly, in order to form an image having a high quality, it is desirable that a scan position of the light beam of the spindle unit is controlled with a high accuracy.
Recently, in image forming apparatuses such as copying machines and printers, opportunities of forming a color image on a medium such as paper have become more frequent. When forming the color image, a plurality of spindle units are used. Hence, in order to form a color image having a high quality, it is desirable to control the plurality of spindle units with a high accuracy so as to rotate in synchronism with each other.
FIG. 1 is a diagram showing an example of a part of a conventional image forming apparatus. In FIG. 1, the image forming apparatus generally includes optical units 101 through 104, photoconductive drums 111 through 114 and developing units 121 through 124 which are respectively provided with respect to yellow (Y), magenta (M), cyan (C) and black (K), and a transport belt 1111 which transports paper 100 in a paper transport path 110. The optical unit 101 optically writes an image on the photoconductive drum 111, and the written image is developed by the developing unit 121, thereby forming a toner image on the photoconductive drum 111. This toner image is transferred onto the paper 100 which is transported. Images are similarly transferred onto the transported paper 100 by the other photoconductive drums 112 through 114. Since the paper 100 in the paper transport path 110 successively makes contact with the photoconductive drums 111 through 114, Y, M, C and K images are successively transferred onto the paper 100 in an overlapping manner, so as to finally transfer a multi-color image on the paper 100.
If the Y image transferred onto the paper 100 by the photoconductive drum 111 and the M image transferred onto the paper 100 by the photoconductive drum 112 do not perfectly overlap, a color printing error occurs and greatly deteriorates the image quality. With respect to the C and B images which are transferred onto the paper 100 by the other photoconductive drums 113 and 114, these C and B images must also overlap perfectly.
When a pitch between two mutually adjacent photoconductive drums is denoted by P mm and a paper transport speed is denoted by V mm/sec, the two images transferred onto the paper 100 by these two mutually adjacent photoconductive drums should overlap perfectly if a time difference T of image transfers made by the two mutually adjacent photoconductive drums satisfies a relationship T=P/S.
For the sake of convenience, it will be assumed that the above described time difference T is an integral multiple of a main scan period C of the photoconductive drum of each optical unit, that is, T=nC, where n is an integer. In this case, main scan start timings of the optical units must be synchronized. In addition, even in a case where the above described time difference T is not an integral multiple of the main scan period C of the photoconductive drum of each optical unit, the main scan start timings of the optical units must still be synchronized with a certain phase error. In the following description, it is assumed for the sake of convenience that the relationship T=nC stands.
When the main scan start timings of two optical units are synchronized, a dot D1 written by one optical unit and a dot D2 written by the other optical unit have the relationship T=nC on the time base as shown in FIG. 2, and the two dots D1 and D2 perfectly overlap on the paper 100. However, if the main scan start timings of the two optical units are not synchronized, a dot D3 written by one optical unit and a dot D4 written by the other optical unit have a relationship T=mC+.alpha. on the time base, where m is an integer and .alpha. is an error quantity, and the two dots D3 and D4 do not overlap perfectly on the paper 100, thereby generating a dot printing error.
FIG. 3 is a diagram for explaining an example of a conventional spindle unit control method. In FIG. 3, each of the optical units 101 through 104 are controlled in the same manner, and thus, only the control with respect to the optical unit 101 will be described. The optical unit 101 includes a laser 131 which emits a laser beam, a spindle unit 132 which is rotated by a motor 133, and a BD detector 134. The spindle unit 132 includes a spindle which is rotated by the motor 133, and a polygonal mirror which is fixed on the spindle. The laser beam from the laser 131 is reflected by the polygonal mirror and scans the corresponding photoconductive drum 111. The BD detector 134 detects a rotary reference position of the polygonal mirror, and outputs a BD signal which indicates the main scan start timing.
A Hall element frequency generator 135 detects a rotational speed (period) of the motor 133, and supplies a FG signal to a phase locked loop (PLL) control circuit 136. The PLL control circuit 136 controls the rotation of the motor 133 by a feedback control based on a reference clock signal RCLK from a reference clock oscillator 139 and the FG signal from the Hall element frequency generator 135.
However, according to the conventional method shown in FIG. 3, no consideration was given whatsoever with respect to the phase relationship of the reference clock signal RCLK and the BD signal. Ideally, the reference clock signal RCLK and BD signals BD1 through BD4 obtained in each of the optical units 101 through 104 should be synchronized as shown in FIG. 4. However, because the phase relationship of the reference clock signal RCLK and the BD signal is actually not controlled, the phase relationship of the reference clock signal RCLK and each of the BD signals BD1 through BD4 becomes indefinite as shown in FIG. 5. For this reason, it was impossible to perfectly synchronize the optical units 101 through 104 so as not to generate the dot printing error described above.
Accordingly, a method has been proposed which supplies the BD signal from the BD detector 134 to the PLL control circuit 136 in place of the FG signal from the Hall element frequency generator 135, as shown in FIG. 6. In FIG. 6, those parts which are the same as those corresponding parts in FIG. 3 are designated by the same reference numerals, and a description thereof will be omitted. In this case, the rotation of the motor 133 is controlled by a feedback control based on the reference clock signal RCLK from the reference clock oscillator 139 and the BD signal from the BD detector 134.
But according to the method shown in FIG. 6, although consideration is given as to the phase relationship of the reference clock signal RCLK and the BD signal, the phase relationship of the reference clock signal RCLK and each of the BD signals BD1 through BD4 becomes essentially fixed. As a result, it is impossible to cope with a change in load, change with time, temperature change and the like in each of the optical units 101 through 104, and it was impossible to perfectly synchronize the optical units 101 through 104 so that the dot printing error described above will not occur.
The speeds (periods) of the reference clock signal RCLK and the FG signal or the BD signal are compared in the PLL control circuit 136, and the motor 133 is driven by a speed control output AFC which is dependent on a result of the speed comparison. In addition, when the rotational period of the motor 133 becomes within .+-.1% of a certain prescribed value, for example, the PLL control circuit 136 starts the phase comparison between the reference clock signal RCLK and the FG signal or the BD signal. For example, a phase control output APC which is dependent on a phase difference between a rising edge of the reference clock signal RCLK and a rising edge of the FG signal or the BD signal is output from the PLL control circuit 136. Accordingly, an energy described by f(VAFC-VAPC) is supplied to the motor 133 from the PLL control circuit 136.
FIG. 7 is a time chart showing the relationship of the reference clock signal RCLK, the FG signal, the phase control output APC, and the speed control output AFC. In FIG. 7, .phi. denotes a phase error between the reference clock signal RCLK and the FG signal. As shown in FIG. 7, an energy component VAPC of the phase control output APC and an energy component VAFC of the speed control output AFC respectively vary depending on the change in load, change with time, temperature change and the like of the optical unit. In other words, the phase error between the reference clock signal RCLK and the FG signal or the BD signal is determined by the characteristic of each individual optical unit and the environment of each optical unit such as the temperature, and the phase error is not always constant.
Hence, when the spindle unit is controlled depending on the comparison result of the comparison between the reference clock signal RCLK and the FG signal or the BD signal, it is impossible to perfectly synchronize a plurality of spindle units so as to prevent the dot printing error, and there was a problem in that the quality of the image becomes deteriorated.
On the other hand, depending on the characteristic of the spindle unit, intervals of the dots formed on the photoconductive drum sometimes deviated along the main scan direction. In addition, the focal distance of the spindle unit with respect to the photoconductive drum differs for each of the spindle units, and the scan width of the photoconductive drum in the main scan direction differs for each of the spindle units. For this reason, the dot printing error was easily generated particularly among the Y, M, C and K images when forming the multi-color image. Therefore, it was desirable to suppress the dot printing error in the main scan direction.